Raid

There are multiple protection options for disk/SSD drives in the SAS SFF bays in Power 750 system unit or drives in 12X attached I/O drawers or drives in disk- only I/O drawers. Although protecting drives is always recommended, AIX/Linux users may choose to leave some or all drives unprotected at their own risk and IBM supports these configurations. IBM i configuration rules differ in this regard, and IBM supports IBM i partition configurations only when disk/SSD drives are protected.

This disk/SSD drive protection can be provided by AIX/IBM i/Linux software or by the disk/SSD hardware controllers. Mirroring of drives is provided by AIX/IBM i/ Linux software. In addition, AIX/Linux supports controllers providing RAID 0, 5, 6, or 10. IBM i integrated storage management already provides striping so IBM i also supports controllers providing RAID 5 or 6. To further augment disk/SSD protection, hot spare capability can be used for protected drives. Specific hot spare prerequisites apply.

An integrated SAS Disk/SSD controller is provided in the Power 750 system unit. It is optionally augmented by a 175 MB write cache and RAID 5 and RAID 6 capability when feature 5679 is added to the configuration. Without feature 5679, the integrated controller supports system mirroring protection for AIX/IBM i/Linux and supports RAID 0 or 10 protection for AIX/Linux. Other disk/SSD controllers are provided as PCI adapters. PCI-X SCSI, PCI-X SAS, and PCIe SAS adapters are supported. PCI Controllers with and without write cache are supported. RAID 5 and RAID 6 on controllers with write cache are supported.

AIX/Linux can use disk drives formatted with 512 byte blocks when being mirrored by the operating system. These disk drives must be reformatted to 528 byte sectors when used in RAID arrays. Although a small percentage of the drive's capacity is lost, additional data protection such as ECC and bad block detection is gained in this reformatting. For example, a 300 GB disk drive when reformatted provides around 283 GB. IBM i always uses drives formatted to 528 byte. IBM Power SSDs are formatted to 528 byte.

RAID 0 (minimum two drives) provides striping without parity for performance, but does not offer any fault tolerance. In data striping, data is broken down into several smaller, equally sized pieces. Each piece is then written to or read from multiple drives. This process increases I/O bandwidth by simultaneously accessing multiple data paths. Because RAID 0 does not offer any redundancy, a single drive failure can result in the loss of all data in a striped set. This means that all of the data on all the drives could be lost if even a single drive fails.

Note that RAID 0 drives can be mirrored by software to provide protection.

RAID 5 (minimum three drives) uses block-level data striping with distributed parity. RAID 5 stripes both data and parity information across three or more drives. Fault tolerance is maintained by ensuring that the parity information for any given block of data is placed on a drive separate from those used to store the data itself. RAID 5 requires N+1 drives to accommodate this parity data, thus the available storage capacity for each array is reduced by one drive to provide protection.

RAID 6 (minimum four drives) uses block-level data striping with dual distributed parity, the same as RAID 5 except RAID 6 uses a second level of independently calculated and distributed parity information for additional fault tolerance. This extra fault tolerance provides data security in the event two drives fail before a drive can be replaced. RAID 6 requires N+2 drives to accommodate the additional parity data.

RAID 10 is RAID 0 plus redundancy. In this type of implementation, an array with an even number of drives is created with mirrored pairs of drives within the array. A RAID 0 stripe set of data is created across the mirrored pairs for performance and for redundancy.

If a protected drive fails, the failing drive can be removed from its hot-plug bay and the drive replaced while the server and partition continue to run. The contents can then be re-created while the system continues to run. Note that until the drive is both replaced and its contents re-created, the protection provided using

IBM United States Hardware Announcement 110-009

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